The successful transplantation of vascularized limbs or even other vascularized blocks of tissue (composite tissue allograft modules; CTA) would have profound implications in reconstructive surgery. CTAs or substructures thereof would be useful for constructing severe musculoskeletal defects. The final goal of these studies is to lay the experimental foundation for the ultimate clinical exploitation of CyA and CTAs. The rat hind limb serves as an excellent vascularized model of a CTA composed of skin, subsutaneous fat, muscle, synovial joints, bone, bone-marrow, and neurovascular structures. The rat limb offers a unique model to study each individual tissue within a well defined histocompatibility system. We have established the first principle necessary for the contemplation of utilizing CTAs combined with cyclosporine clinically: namely, that a minimal CyA dose is effective for preventing rejection (1). Secondly, we have also established that composite tissue allografts transplanted across this major histocompatibility barrier can demonstrated indefinite survival with moderate doses of cyclosporine with no inherent toxicity (2). Thus, we believe that the clinical potential for CTAs is even more promising today than we had originally envisioned. We intend to further study methods aimed at achieving indefinite or long-term allograft survival in combination with the use of CyA. The immune status of long-term surviving, chimeric, and apparently tolerant CTA recipients (2,3) will be characterized. Unique rejection processes will be further delineated histothologically. A new dose-response curve will be established across a much stronger histocompability barrier (ACI-greater than LEW; Rt-1 a-greater than 1) to convincingly confirm cyclosporine's potential for preventing CTA rejection at relatively low doses. We propose to characterizing the possible development of lymphoid chimerism and thus potential graft versus host disease in this strong fully allogeneic CTA model with sensitive fluorescence activated cell sorter analysis.